They collected evidences, which indicated his intentions of going to Syria. They also retrieved a letter, which insisted that his parents joined him in the caliphate Syria. He also said,"Western societies are getting more immoral day by day," he wrote in the letter, adding that he did not want to remain in the United States where his taxes would go toward killing fellow Muslims. "I do not want my kids being exposed to filth like this."
Khan is not alone, there were 9 more such cases where youth (the oldest being 29 years old), including women, wanted to go to Syria to join ISIS. In some of these cases, undercover investigating agents in plain clothes collected evidences of their intentions. Following this, they were stopped at the airport.
The Justice Department, however is more concerned about the fact that Americans are going to Syria, receiving training and coming back to the USA and attacking on behalf of the trrop. However, there have been no active plots against the United States by these organizations.
Intelligence agencies estimate that more than 100 Americans have gone or tried to go in Syria. About a dozen are said to be fighting in the battlefield. It is not illegal to travel to Syria, or any country. Nearly all the people who have been arrested have been charged under a federal statute that bars providing material support to terrorists. To gain a conviction on such a charge, the government must prove that the person intended to provide some type of aid to the group.
If proved, the charges can lead to a minimum of 15 years of imprisonment and a $250,000 fine.